The present invention is directed toward improved thermal expansion and thermal shock resistance of cordierite structures. It has been found that by lengthening the time of and/or increasing the temperature of the heat treatment of the calcined kaolin prior to mixing this component as part of the cordierite batch, provides a combination of properties to the final fired structure heretofore unobserved. These improvements are manifested in the cordierite sintered structure by a combination of factors, such as lower thermal expansion coefficient, higher thermal shock resistance, and greater cordierite alignment as evidenced by the I ratio.
Structures, commonly shaped as a honeycomb, made from cordierite are used as substrates which are subsequently coated with a high surface area washcoat, such as alumina, catalyzed and subsequently placed in the path of the exhaust effluent of internal combustion engines. Other useful products made from such material can be gainfully employed as filters for fluids such as diesel particulate filters and ultrafiltration devices, or as substrates for woodstove combustors or DeNOx catalytic converters for power plants.
Those skilled in this art know that typically raw materials such as talc, kaolin clay, alumina, silica, aluminum hydroxide, various inorganic and organic binders, and water can be advantageously mixed to form a batch. The batch is plasticized and subsequently extruded to form greenware. The greenware is then fired to form a cordierite sintered structure.
It is known by those in this art that mullite may be formed during sintering and that if mullite is preferentially oriented within a certain plane of the structure during sintering, that the cordierite in the structure may be preferentially oriented, resulting in a lower thermal expansion in the resultant structure. The orientation is often discussed in terms of I ratio. The I ratio is defined as the preferred orientation axis divided by the other two axes in the three dimensional planes of the crystalline structure. In the case of cordierite, the preferred I ratio axis is the c-axis. Further discussion of the attributes of orientation may be found in "Thermal Expansion of Extruded Cordierite Ceramics" by Irwin M. Lachman, Rodney D. Bagley, and Ronald M. Lewis, American Ceramic Society Bulletin, Vol. 60, No. 2, February 1981, the disclosure of which is herein incorporated by reference.
U.S. Pat. No. 4,280,845 discloses the effects of orientation and firing schedules on the resultant durability of the cordierite structure. There, a full discussion flushes out the various advantages that accrue due to orientation, particle size, thermal expansion, and the effect of various phases found in the fired cordierite. Reference is made to a calcined kaolin component to the batch, however, no reference is made therein to processing the kaolin in a manner to produce a certain percentage of mullite prior to the firing of the cordierite body.
A continual problem in this art is the variation of batch parameters to enable the production of consistent structural properties when the ceramic process is put into manufacturing production scale-up. Prior to the present inventive insights, the calcined kaolin in this batch has been routinely added without notice of the variations that may be produced by its inclusion. The present invention, however, carefully considers this component and nails down the sensitivity of the final sintered product to the treatment of the calcined kaolin prior to mixing this component into the batch. Essentially, the modifications made thereto introduce to the batch a calcined kaolin of a different nature, the properties of which manifest themselves in the final fired structure.